Upon propagating through a material, electron beams gradually deliver part of their energy to the material by exciting orbital electrons in the material to give rise to chemical reaction or generate secondary electrons and x-rays and thus decelerate while the electron beams are scattered to progress in diverse directions or diffuse. This tendency becomes outstanding especially in a high density material such as a solid. Electron beams having such nature have already been used in the curing of print ink, paint, and removers for stripping sticky substances. On the other hand, an apparatus for irradiating electron beams generally includes an electron emitting means and an electron accelerating means. The prior art apparatus used in the curing of resins is generally designed so as to irradiate electron beams over a large area for increased productivity. Apparatus operable at relatively low accelerating voltage adopt the so-called curtain type consisting of an electron emitting and accelerating means for producing a wide band of electron beams while apparatus operable at relatively high accelerating voltage adopt the so-called scan type consisting of an electron emitting and accelerating means for producing a narrow strip of electron beams and an electron scanning means for distributing the beams over a larger area.
Electrons are emitted and accelerated in vacuum while they are irradiated to an article in an atmospheric environment that assures ease of continuous treatment from the productivity standpoint. Most often, the atmospheric environment and the vacuum are partitioned by a window in the form of a thin metal foil by which electron beams are transmitted so that electron beams are taken from the vacuum to the atmospheric environment. When electron beams are transmitted by the window, they are substantially scattered. Such diffusion of electron beams after window passage is not problematic because electron beams are to be irradiated over a large area. When electron beams are irradiated to a very thin member such as optical fiber, however, the efficiency of irradiation is very low. On the other hand, when electron beams are irradiated in vacuum, the technique of converging electron beams under the action of a magnetic field has been established and utilized in electron beam welders or the like. If electron beams are continuously irradiated to a length of resin-coated optical fiber in vacuum, it is difficult to maintain the degree of vacuum constant and there is a risk that the resin can foam and scatter away.
JP-B 5-50454 discloses the electron beam curing of optical fiber coating materials, but does not refer to the irradiation efficiency of electron beams. The technique of continuously irradiating electron beams to an optical fiber under atmospheric pressure at a high efficiency has not been established.
Current optical fibers include a variety of types such as quartz glass, multi-component glass and plastic fibers. Among these, quartz glass type optical fibers are vastly used in a wide variety of applications because of their light weight, low loss, durability and high transmission capacity. Since quartz glass type optical fibers are as thin as having a diameter of 125 μm typical to the most commonly used fibers, they are vulnerable to failure even with faint flaw and increase the transmission loss by external stresses as by bending. For this reason, the optical fibers are generally provided with a resin coating of two layers, a relatively soft primary coating layer and a secondary coating layer enclosing the primary coating layer. Most often, immediately after drawing from a melt, a bare optical fiber is coated with a liquid resin by a die coating technique or the like, which is cured with heat or radiation (typically UV radiation). The secondary coating is applied and cured simultaneously with or subsequent to the application and curing of the primary coating. The thus coated optical fiber, which is generally designated coated optical fiber or simply optical fiber, is colored with ink for identification. A tape element is fabricated by bundling several, typically four or eight, coated optical fibers and coating the bundle with a taping material or liquid resin, followed by heating or radiation (typically UV) exposure for curing.
Typical of the coating material are urethane acrylate base UV-curable resin compositions. As disclosed in JP-B 1-19694 and Japanese Patent Nos. 2,522,663 and 2,547,021, liquid UV-curable resin compositions comprising a urethane acrylate oligomer, a reactive diluent, and a photo-polymerization initiator are known.
In the recent manufacture of optical fibers, the drawing speed of optical fibers has increased for productivity improvement purposes. The energy per unit time required to cure the resin coating material has to be increased. However, the customary UV curing technique is in the state that any output increase devised for UV lamps has not caught up with the progress. Then a number of UV irradiating lamps must be arranged in series. The production speed is limited by the dimensions of a space where lamps can be installed.
It is generally believed that electron beam curing is energy efficient as compared with UV curing. This is true only when an article to be irradiated with electron beams has a large area as in resin curing in the case of coated paper and printing ink, so that electron beams, even when diffused, may impinge anywhere on the article. If the prior art electron beam irradiating apparatus of the curtain type is used in order to irradiate electron beams to a thin elongated wire (e.g., optical fiber), the proportion of effective electrons impinging on the thin wire is very low because of the substantial scattering of electron beams upon transmission through the metal foil, even when the direction of a band of electron beams is matched with the longitudinal direction of the thin wire. This leads to the problem of very low energy efficiency. On use of the electron beam irradiating apparatus of the scan type, even if electron beams could be kept stationary on the thin wire without scanning, there also arises the problem of low energy efficiency because of the substantial scattering of electron beams upon transmission through the metal foil. In the event where a polyethylene coated electric conductor is subject to electron beam crosslinking, the problem can be overcome by moving the conductor in turns so that the conductor passes many times the irradiating chamber. This approach is not employable in the electron beam curing of a liquid coating composition on an optical fiber, because bending of the coated fiber prior to complete cure can cause the coating to be damaged or stripped.
Another problem is that when electron beams are irradiated to a coating material on optical fibers which are doped with germanium, the germanium can be altered, resulting in an undesirably increased transmission loss.